This invention relates in general to the field of subterranean exploration and, more particularly, to a pantograph underreamer.
Underreamers are generally used to form an enlarged cavity in a well bore extending through a subterranean formation. The cavity may then be used to collect resources for transport to the surface, as a sump for the collection of well bore formation cuttings and the like, or for other suitable subterranean exploration and resource production operations. Additionally, the cavity may be used in well bore drilling operations to provide an enlarged target for constructing multiple intersecting well bores.
One example of an underreamer includes a plurality of cutting blades pivotally coupled to a lower end of a drill pipe. Centrifugal forces caused by rotation of the drill pipe extend the cutting blades outward and diametrically opposed to each other. As the cutting blades extend outward, the centrifugal forces cause the cutting blades to contact the surrounding formation and cut through the formation. The drill pipe may be rotated until the cutting blades are disposed in a position substantially perpendicular to the drill pipe, at which time the drill pipe may be raised and/or lowered within the formation to form a cylindrical cavity within the formation.
Conventional underreamers, however, suffer several disadvantages. For example, the underreamer described above generally requires high rotational speeds to produce an adequate level of centrifugal force to cause the cutting blades to cut into the formation. An equipment failure occurring during high speed rotation of the above-described underreamer may cause serious harm to operators of the underreamer as well as damage and/or destruction of additional drilling equipment.
Additionally, density variations in the subsurface formation may cause each of the cutting blades to extend outward at different rates and/or different positions relative to the drill pipe. The varied positions of the cutting blades relative to the drill pipe may cause an out-of-balance condition of the underreamer, thereby creating undesired vibration and rotational characteristics during cavity formation, as well as an increased likelihood of equipment failure.
Accordingly, a need has arisen for an improved underreamer that provides increased control of subterranean cavity formation. The present invention provides a pantograph underreamer that addresses shortcomings of prior underreamers.
According to one embodiment of the present invention, an underreamer for forming a cavity within a well bore includes a housing rotatably disposed within the well bore. The underreamer also includes an actuation rod slidably positioned in the housing. The underreamer further includes a plurality of cutter sets nested together around the actuation rod while the cutter sets are in a retracted position where each cutter set includes a first end pivotally coupled to the housing and a second end pivotally coupled to the actuation rod. The cutter sets are also pivotally coupled together. An axial force applied to the actuation rod is operable to slide the actuation rod relative to the housing and extend the cutter sets radially outward relative to the housing from the retracted position to form the cavity when the housing is rotated.
According to another embodiment of the present invention, a method for forming a cavity within a well bore includes positioning an underreamer within the well bore. The underreamer includes a housing and an actuation rod. The actuation rod is slidably positioned in the housing. The underreamer further includes a plurality of cutter sets nested together around the actuation rod while the cutter sets are in a retracted position where each cutter set includes a first end coupled to the housing and a second end coupled to the actuation rod. The method further includes applying an axial force to the actuation rod and extending the cutter sets radially outward from the retracted position relative to the housing in response to movement of the actuation rod relative to the housing from the applied force. The method further includes rotating the underreamer within the well bore to form the cavity.
The invention provides several technical advantages. For example, according to one embodiment of the present invention, an axial force is applied to an actuation rod of the underreamer to cause outwardly directed movement of cutter sets into a subterranean formation. The axial force applied to the actuation rod may be varied to produce corresponding varying pressures on the formation by the cutter sets. Thus, the present invention may be used to accommodate a variety of formation densities and compositions. Additionally, decreased rotational speeds of the underreamer may be used to form the cavity, thereby substantially reducing or eliminating hazards associated with high speed rotating mechanisms.
Another technical advantage of the present invention includes substantially reducing or eliminating out-of-balance conditions resulting from rotation of the underreamer within a well bore. For example, according to one embodiment of the present invention, an end of each of the cutter sets is coupled to the actuation rod, thereby resulting in substantially uniform extension and increased precision of each of the cutter sets relative to the underreamer housing. Thus, out-of-balance conditions caused by varying positions of cutting blades are substantially reduced or eliminated.
Other technical advantages will be readily apparent to one skilled in the art from the following figures, descriptions, and claims.